Close all plot windows of a blocking show() on Ctrl+C

Addresses the Qt part of matplotlib#23385.

It appears that `qapp.quit()` does not automatically close the windows
of the app. We therefore do it explicitly.

A unit test for this would be quite complex. Test this
by hand by Ctrl+C in an interactive shell.

lib/matplotlib/backends/qt_compat.py

@@ -215,6 +215,8 @@ def _may_clear_sock(*args):
     def handle(*args):
         nonlocal handler_args
         handler_args = args
+        for window in qapp.allWindows():
+            window.close()
         qapp.quit()
 
     signal.signal(signal.SIGINT, handle)